Chapter 8 Metamorphic Rocks • Metamorphic rock forms from protolith . • Metamorphism does not involve melting, solid-state process • Metamorphic texture, the grains in the rock have grown in place and interlock. • Metamorphic rocks form very slowly • Many different processes of formation: o Recrystallization: changes the shape and size of grains without changing the identity of the mineral constituting the grains. o Phase Change: transforms a grain of one mineral into a grain of another mineral. o Metamorphic reaction/neocrystallization: growth of new mineral crystals that differ from those of the protolith, one or more chemical reactions takes place. o Pressure Solution: a rock is squeezed more strongly in one direction than in others at relatively low pressures and temperatures in the presence of water. • Rocks undergo metamorphism when they are subjected to heat, pressure, differential stress, and/or hydrothermal fluids. • Near the earth’s surface, minerals with relatively open crystal structures are stable ---- subjected to extreme pressure, denser minerals form. <phase change/neocrystallization • Rocks feel the effects of pressure and temperature that effect metamorphism during mountain building. • Differential Stress: when a material is squeezed or stretched unequally from different sides --- the push or pull in one direction differs in magnitude from the push or pull in another direction --- two different kinds: o Normal stress: pushes or pulls perpendicular to a surface. push=compression, pull=tension --- compression flattens a material, tension stretches a material o Shear Stress: moves one part of a material sideways, relative to another.

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• When rocks are subjected to differential stress at elevated temperatures and pressures they can change shape without breaking. • As a rock changes shape, the internal texture of a rock also changes --- results in preferred mineral orientation --- platy grains lie parallel to each other and elongate grains align in the same direction. o Platy and elongate grains are not the same length in all directions (inequant ----eqaunt is the opposite, grains are the same length) • Metamorphic reactions usually take place in the presence of hydrothermal fluids, because water occurs throughout the crust --- hydrothermal fluids consist of hot water, steam, and supercritical fluid. • Hydrothermal fluids are chemically active, they are able to chemically react with rock • Metasomatism: when rock’s chemical composition changes because of hydrothermal fluids • Metamorphic rocks are classified into two different classes: Foliated and Nonfoliated o Foliation: repetition of planar surfaces or layers in a metamorphic rock --- can give a rock a striped or streaked appearance in an outcrop ---can give them ability to split into thin sheets --- contains inequant mineral crystals that are aligned parallel to each other or because the rock has alternation dark and light colored layers o Foliated rocks can be distinguished from each other according

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